Optical fiber and optical waveguide mode converters are well known and come in a variety of forms. They operate typically by transforming an input mode, usually a fundamental mode, into a higher order mode, or vice versa. An especially attractive mode converter device comprises a long period grating (LPG) formed in an optical fiber. See for example, U.S. Pat. No. 6,768,835, and T. Erdogan, “Fiber grating spectra,” J. Lightwave Technology vol. 15, p. 1277 (1997).
These mode converters operate with a single mode input, and typically a single mode output. Propagating light in more than one mode at a time, and controllably changing the mode of more than one mode at a time, would be an attractive goal, but to date not achieved.